Sorry for delay; thanks for the comments.
I do not believe it is due to badly mixed beer when batch priming, as it seems the beers get fizzier over time rather than one being fizzy and the next one being flat. The chance that I'm drinking them in the order they were bottled seems too small to consider!
I believe all the cases were batch-primed not adding sugar to each bottle. Here are 3 examples... the stout has only just started to show this behaviour and it's particularly annoying as who wants a fizzy stout?!
I leave my beers to prime for at least 2 weeks before trying them and generally at that point they have the desired level of expected carbonation. More recently I have conditioned them in my fridge at a a controlled 20-22C
|Youngs Red IPA||1048||1014||4.8 (came with kit)|
|St. Peter Honey Porter||1042||1012||4.1|
I just finished the Red IPA the other day and it had got so fizzy I had to de-carbonate it before it was drinkable or even pourable. This and the porter were both bottled in November and white the porter was initially good, it too is now quite fizzy. Ironically the head has got much more long-lasting as it's aged but I'm having to de-carbonate it and I have a case left to drink. I can chill it more but even so.
I'd been advised that once yeast stalled/stuck this meant it could only then cope with simple sugar. But it does give the appearance that bottling after priming, the yeast is kicking off. Maybe I should take a gravity reading on one to see if this is really the case... if the gravity dropped more than a point or so, the priming sugar can't be to blame.